There have been known vehicle control systems having an engine and a hydro-mechanical transmission (HMT) which transmits engine power from an input shaft to an output shaft through a mechanical transmission unit and a hydrostatic transmission unit (e.g., Japanese Patent Kokoku Publication No. 62-31660). Since mechanical power can be transmitted with high efficiency, the transmission (HMT) provided for this control system is designed to convert only part of mechanical power into hydraulic power, so that it can achieve high transmission efficiency. Such a transmission is considered to be an ideal transmission for vehicles subjected to wide load variations such as bulldozers and wheel loaders and is, in fact, employed in some vehicles.
In a typical hydro-mechanical transmission (HMT), variable speed characteristics are achieved by a planetary gear train. More concretely, the transmission is arranged as follows. Of three elements of the planetary gear train (i.e., the sun gear, the carrier provided with planetary gears, and the ring gear), a first element is coupled to the input shaft, a second element is coupled to the output shaft, and a third element is coupled to a hydraulic pump or hydraulic motor. The rotational speed of the hydraulic pump or hydraulic motor is varied thereby changing the rotational speed of the output shaft.
In the known art, there are basically two types of HMTs. One is an “output-split type” where a hydraulic pump or hydraulic motor, which is connected to another hydraulic pump or hydraulic motor coupled to the planetary gear train by means of a hydraulic circuit, is coupled to the input shaft of the transmission so as to obtain a constant speed ratio. The other is an “input-split type” where a hydraulic pump or hydraulic motor, which is connected to another hydraulic pump or hydraulic motor coupled to the planetary gear train by means of a hydraulic circuit, is coupled to the output shaft of the transmission so as to obtain a constant speed ratio. Further, the output-split type and input-split type are respectively classified into six types according to which of the three elements of the planetary gear train is coupled to the hydraulic pump, hydraulic motor or input/output shafts and, in total, 12 types are available as basic combinations.
The conventional output-split type HMT and input-split type HMT will be respectively described in more detail.
FIG. 15(a) shows a schematic structural diagram of an output-split type HMT. In this output-split type HMT 100, a first gear 103 is secured to an input shaft 102 to which power from an engine 101 is input. A second gear 104 meshing with the first gear 103 is secured to a shaft 105a of a first pump/motor 105. Secured to the input shaft 102 is a sun gear 107 of a planetary gear train 106. A plurality of planetary gears 108 are disposed so as to mesh with the periphery of the sun gear 107. Each planetary gear 108 is axially supported by a planetary carrier 109 to which an output shaft 110 is secured. A ring gear 111 meshes with the periphery of the planetary gear set 108. Meshing with the periphery of the ring gear 111 is a third gear 112 which is, in turn, secured to a shaft 113a of a second pump/motor 113. In this arrangement, the first pump/motor 105 is hydraulically connected to the second pump/motor 113 by a piping 114.
In such a system, when the rotational speed of the second pump/motor 113, that is, the rotational speed of the ring gear 111 is zero, hydraulically transmitted power becomes zero so that all power is transmitted through the mechanical unit. On the basis of the rotational speed of the output shaft 110 at that time, the operation of this system will be described below.
(1) When increasing the rotational speed of the output shaft 110, the second pump/motor 113 receives motive power through the medium of hydraulic pressure and is activated to increase the rotational speed of the output shaft 110. At that time, the first pump/motor 105 serves as a pump whereas the second pump/motor 113 serves as a motor, so that energy is transmitted from the first pump/motor 105 to the second pump/motor 113 through the medium of hydraulic pressure. Then, the horsepower transmitted by hydraulic power becomes plus (+) as indicated by line A-B in FIG. 15(b) and the hydraulic power flows in a forward direction, i.e., from the input shaft 102 toward the planetary gear train 106.
(2) When reducing the rotational speed of the output shaft 110, the second pump/motor 113 receives motive power from the planetary gear train 106 and rotates in a direction opposite to that of the case (1). At that time, the second pump/motor 113 serves as a pump whereas the first pump/motor 105 serves as a motor, so that energy is transmitted from the second pump/motor 113 to the first pump/motor 105 through the medium of hydraulic pressure. Then, the horsepower transmitted by hydraulic power becomes minus (−) as indicated by line A-C in FIG. 15(b) and the hydraulic power flows in a reverse direction, i.e., from the planetary gear train 106 toward the input shaft 102.
FIG. 16(a) shows an input-split type HMT 200 in which the planetary gear train 106 is disposed on the side of the input shaft 102 whereas the first pump/motor 105 is disposed on the side of the output shaft 110. In FIG. 16(a), the parts that are substantially equivalent or function substantially similarly to those of the transmission 100 shown in FIG. 15(a) are indicated by the same numerals as in FIG. 15(a), and a detailed explanation of them is skipped herein.
The input-split type transmission 200 is constructed as follows.
(1) When increasing the rotational speed of the output shaft 110, the second pump/motor 113 serves as a motor while the first pump/motor 105 serves as a pump, so that energy is transmitted from the first pump/motor 105 to the second pump/motor 113 through the medium of hydraulic pressure. Then, the horsepower transmitted by hydraulic power becomes minus (−) as indicated by line A-D in FIG. 16(b) and the hydraulic power flows in a reverse direction, i.e., from the output shaft 110 toward the planetary gear train 106.
(2) When reducing the rotational speed of the output shaft 110, the second pump/motor 113 receives motive power from the planetary gear train 106 and rotates in a direction opposite to that of the case (1). At that time, the second pump/motor 113 serves as a pump whereas the first pump/motor 105 serves as a motor, so that energy is transmitted from the second pump/motor 113 to the first pump/motor 105 through the medium of hydraulic pressure. Then, the horsepower transmitted by hydraulic power becomes plus (+) as indicated by line A-E in FIG. 16(b) and the hydraulic power flows in a forward direction, i.e., from the planetary gear train 106 toward the output shaft 110.
As such, in both of the output-split type and input-split type transmissions, energy flows in forward and reverse directions occur in the speed increasing area and the speed reducing area. The energy transmission efficiency in this case will be hereinafter examined, taking the output-split type HMT 100 shown in FIG. 15 for example. Herein, the transmission efficiency of the mechanical unit is 95% and the transmission efficiency of the hydrostatic unit is 80% (Generally, where pump-motors are used, transmission efficiency is low). For easy comparison, assume that the amount of engine power is 1.0 and one third the engine power is input to the hydrostatic unit.
FIG. 17(a) shows the case where hydraulic power flows in the forward direction. Specifically, one third (0.333 part) the energy output from the engine 101 flows to the hydrostatic unit for increasing speed. Transmitted to the output shaft 110 are 0.633 (=(1−⅓)×0.95) part of energy from the mechanical unit and 0.267 (=0.333×0.8) part of energy from the hydrostatic unit. As a result, the overall efficiency becomes 0.9 (=0.633+0.267). The case where hydraulic power flows in the reverse direction is shown in FIG. 17(b). In this case, 1.267 (=1+0.267) parts of energy are input to the mechanical unit and 1.20 (=1.267×0.95) parts of energy are transmitted, so that the overall efficiency is 0.870 (=1.20−0.333).
As just described, when hydraulic power flows in the reverse direction, a large flow of energy occurs in each element, resulting in poor efficiency. In other words, a forward flow of hydraulic energy is better than a reverse flow of hydraulic energy. As seen from FIGS. 17(a) and 17(b), if part of energy flows in the reverse direction, the amount of energy that passes through the mechanical unit will increase, and therefore, there arises a need to increase the size of the planetary gear train, which leads to a disadvantage in economical efficiency.
As an attempt to solve the problems of the prior art output-split type HMT and input-split type HMT, there has been proposed a transmission capable of serving as an output-split type HMT when the rotational speed of the output shaft is increased and as an input-split type HMT when the rotational speed of the output shaft is reduced (Hereinafter, this proposed transmission is referred to as “output-split/input-split switching type HMT”). The output-split/input-split switching type HMT has several advantages. For instance, the horsepower transmitted by hydraulic power can be kept to be plus irrespective of the rotational speed of the output shaft, so that hydraulic power can be allowed to constantly flow in the forward direction and increased energy efficiency can be achieved in all speed regions ranging from the low speed region to the high speed region.
In a vehicle control system having the output-split HMT 100, a vehicle control system having the input-split HMT 200 and a vehicle control system having the output-split/input-split switching type HMT, the rotational speed of the engine 101 depends on the opening of the throttle and load torque. The load torque varies according to the rotational speed ratio of the output shaft to the input shaft of the transmission 100 (200) (=output shaft rotational speed/input shaft rotational speed: this ratio is hereinafter referred to as “speed ratio”). Accordingly, as the speed ratio increases, the engine speed decreases, and as the speed ratio decreases, the engine speed increases. The speed ratio is varied by controlling the tilt angles of the swash plates of the first and second pump/motors 105, 113.
In the above vehicle control systems, the engine is brought into a running condition most suitable for the opening of the throttle in such a way that an engine speed at which the engine can be operated in an optimum running condition is set as a target engine speed and the actual speed of the engine is made close to the target engine speed by speed ratio control in which the tilt angles of the swash plates are controlled.
In addition, the above vehicle control systems are provided with a decelerator for arbitrarily restricting the speed ratio within an upper limit that is set based on the lever position of a lever for setting a maximum speed. By operating the decelerator to limit the speed ratio, the output shaft rotational speed (=vehicle speed) of the transmission 100 (200) is reduced.
However, the above vehicle control systems have not proved successful in that although restriction of the speed ratio through operation of the decelerator causes the rotational speed of the output shaft (i.e., vehicle speed) to decrease, the engine speed increases, which gives the operator a feeling of operational disorder.
The present invention is directed to overcoming the foregoing shortcomings and a primary object of the invention is therefore to provide a vehicle control system capable of decreasing the rotational speed (i.e., vehicle speed) of the output shaft of the transmission in response to decelerating operation while decreasing engine speed, so that the operator does not feel strangeness during operation.